A known transmission ratio variable mechanism disposed between a steering wheel and gear device such as rack and pinion mechanism for varying a transmission ratio between a steering amount of the steering wheel and a turning amount of vehicle wheels is disclosed in Japanese Patent Laid-Open Publication No. H10-324263.
The known transmission ratio variable mechanism includes a direct connection means. The direct connection means restrict a relative rotation between an input shaft and output shaft when reverse input or an abnormality of a motor. Accordingly the transmission ratio variable mechanism prevent a deviation between the steering amount of the steering wheel and turning amount of the wheels in case a torque is applied from an output shaft of the transmission ratio variable mechanism due to the reverse input or in case of the stop of motor rotation due to the abnormality.
Notwithstanding, with the construction of the known transmission ratio variable mechanism, assembling of the direct connection means is complex. In addition, a detection means for detecting the abnormality of the motor and the reverse input and a control means for commanding the operation of the direct connection means are additionally required for operating the connection means.
This increases the number of parts and manufacturing costs. Further, the known transmission ratio variable mechanism with the foregoing construction has a drawback that it requires a response time from the detection of the abnormality of the motor or the reverse input by the detection means until the operation of the direct connection means is completed by commanding the operation of the direct connection portion by the control means.
A need thus exists for a transmission ratio variable mechanism which can maintain a relation between a steering amount of a steering wheel and a turning amount of wheels even when a reverse input from the turning wheels side is input or an abnormality of the motor are generated without providing a direct connection means which requires an additional detection means and a control means.